A potentiometer instrument for measuring the potential (or voltage) in a circuit taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The sliding tap of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding tap and the unknown potential; the deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology.
Benson John Lossing, ed. The New Popular Educator (London, England: Cassell & Company Limited, 1891)